Republicans marched out of their convention intoxicated with the sensation of victory. President Bush, the "war president", was the most honest, moral, decisive, and strongest leader in the world. (The unvarying encomiums eerily echoed those of the brainwashed soldiers about the sleeper agent in The Manchurian Candidate: "Raymond Shaw is the kindest, warmest, bravest, most wonderful human being I've ever known in my life.")

After Bush's defiant speech - "Nothing will hold us back!" - his lead was reported by Time magazine to have climbed to 11 points, which was inhaled like pure oxygen by the Republican cadres. (Both John Kerry's and Bush's internal polls gave Bush only a four-point lead.)

Kerry seemed to be reeling in retreat. His disciplined campaign management had suppressed criticism of Bush, supposedly on the basis that swing voters are attracted by vague swirls of optimism. But the effect was that voters remained confused about the contrast between the candidates and Kerry's commitments. Kerry had delayed defending himself against the torpedoes of falsehood fired at his heroic military record by the Orwellianly named Swift Boat Veterans for Truth.

Perhaps his gravest self-inflicted wound was replying to Bush's challenge to answer whether he would still have voted for the war resolution on Iraq, knowing what he does today. Kerry said he would and tangled himself in a thicket of sticky nuance.

Bush could hardly believe that Kerry had fallen for the gambit. This sucker would buy a bridge in Brooklyn. The triumphant Republicans felt unrestrained in delivering blows to the prone Kerry. Dick Cheney announced that a vote against Bush was tantamount to a vote for a terrorist attack: "If we make the wrong choice then the danger is that we'll get hit again."

On the day that former President Clinton had his heart surgery, Cheney attacked him as weak on terrorism, and for good measure set upon Ronald Reagan too. The venerated Reagan had served his purpose as an icon at the convention, but now he was unceremoniously thrown overboard.

Only Bush was tough enough. Bush, adopting the tone of the fraternity house president he once was, sarcastically derided Kerry: "No matter how many times Senator Kerry flip-flops, we were right to make America safer by removing Saddam Hussein from power."

In fact, on the third day of the Republican convention, Kerry had given a penetrating and highly specific speech on the war on terrorism and Iraq, detailing how Bush's strategy amounted to a series of catastrophic blunders. "When it comes to Iraq," he said, "it's not that I would have done one thing differently, I would have done almost everything differently."

Kerry's speech was pointedly ignored by Bush who, with Cheney, rained a steady fire of ridicule down on Kerry. Meanwhile, the report on Iraq by the Royal Institute of International Affairs was buried in the back pages. "Iraq could splinter into civil war and destabilise the whole region if the interim government, US forces and United Nations fail to hold the ring among factions struggling for power." Civil war, the institute said, was "the most likely outcome". Kerry remarked that because of Bush's errors "terrorists have secured havens in Iraq that were not there before". The New York Times reported that Fallujah and many other cities in the Sunni triangle are under the control of Islamist insurgents. But Bush steadfastly refused to engage Kerry in debate. A report chronicling the undermining of the war against terrorism by James Fallows in The Atlantic, in which numerous military officials described how Afghanistan became a "sideshow" as resources were siphoned to Iraq, received almost no attention. "Our strategy is succeeding," Bush told his jubilant rallies.

Bush campaigns before the faithful; distressing facts are dismissed with sarcasm and ideology is implacable. Yet at this moment of disdain a discovery that cast light on Bush's character suddenly emerged, having the potential to alter the momentum of the campaign.

On Wednesday, the Boston Globe published documents proving that Bush, whose spotty record in the National Guard was always mysterious, "fell well short of meeting his military obligation". Maj Gen Paul A Weaver Jr., who retired in 2002 as the Pentagon's director of the Air National Guard, was quoted: "It appears that no one wanted to hold him accountable."

That night, CBS's 60 Minutes broadcast the first interview with former Texas lieutenant governor Ben Barnes, who explained how he contrived to get young George his safe posting in the "champagne unit" of the Texas Air National Guard during the Vietnam war. The programme also revealed further documents showing he never fulfilled his service.

Abruptly, the Republican marchers stumble as Kerry is galvanised. "His miscalculation was going to war without planning carefully and without the allies we should have had," he said yesterday. Meanwhile in the White House, aides anxiously wonder how to explain the president's haunted past and his long years of hiding it and who will have the task of facing the cameras.

· Sidney Blumenthal, a former senior adviser to President Clinton, is Washington bureau chief of salon.com